Use of designer GPCRs to study GPCR regulation of key metabolic pathways Armbruster et al. (PNAS 104, 5163-8, 2007) first described a set of muscarinic receptor-based designer GPCRs which are now generally referred to as DREADDs ('designer receptors exclusively activated by designer drugs'). These designer receptors are unable to bind the endogenous muscarinic receptor agonist, acetylcholine, due to two single point mutations introduced into the transmembrane receptor core. Importantly, DREADDs can be efficiently activated by a compound called clozapine-N-oxide (CNO), an agent that is otherwise pharmacologically inert. The first DREADDs that were developed represent GPCRs that selectively activate G proteins of the Gq or Gi family, respectively. We subsequently generated additional DREADDs endowed with different coupling properties, including a Gs DREADD and a functionally promiscuous DREADD (Guettier et al., PNAS 106, 19197-202, 2009). More recently, we generated an M3R-based DREADD that is uncoupled from G proteins but retains arrestin-dependent signaling (Nakajima and Wess, Mol Pharmacol 82, 575-82, 2012). Analogously, we recently designed an M3R-based DREADD that shows the opposite coupling profile: lack of arrestin recruitment but efficient coupling to Gq-type G proteins (Hu et al., J Biol Chem 291, 7809-20, 2016) (for a recent review see: Wess J. Use of Designer G protein-coupled receptors to dissect metabolic pathways. Trends Endocrinol Metab. 2016 Sep;27(9):600-3). We are currently in the process of expressing DREADDs with different coupling properties in various metabolically relevant cell types. These cell types include adipocytes, pancreatic beta-cells, skeletal muscle cells, hepatocytes, and certain neuronal subpopulations of the hypothalamus. Preliminary results indicate that CNO treatment of some of these mutant mouse strains has pronounced effect on glucose and energy homeostasis (unpublished results). Hepatic Gi signaling is important for maintaining euglycemia One of the major pathophysiological features of type 2 diabetes is an elevation in hepatic glucose production (HGP). Excessive signaling through hepatic, Gs-linked glucagon receptors critically contributes to pathologically elevated HGP. To test the hypothesis that this metabolic impairment can be counteracted by enhancing hepatic Gi signaling, we used a chemogenetic approach to selectively activate Gi-type G proteins in mouse hepatocytes in vivo. Surprisingly, activation of hepatic Gi signaling did not reduce hepatic glucose output but triggered a pronounced increase in HGP and severe impairments in glucose homeostasis. Mice lacking functional Gi-type G proteins in hepatocytes showed the opposite phenotype and were protected against the metabolic deficits caused by the consumption of a high-fat diet. We also delineated a novel signaling cascade that links hepatic Gi signaling to increased HGP. Our data support the new concept that blockers of hepatic Gi-coupled GPCRs may prove beneficial as novel antidiabetic drugs. (Rossi M, Zhu L, McMillin SM, Pydi SP, Cu Y, Lee RJ, Kaneto H, Birnbaum MJ, Ma Y, Rotman Y, Liu J, Cyphert TJ, Finkel T, McGuinness OP, Wess J. Hepatic Gi signaling as a critical novel regulator of whole body glucose homeostasis. J Clin Invest 128, 746-759, 2018) Beta-arrestin-2 is a major negative regulator of hepatic glucagon receptors An increase in hepatic glucose production (HGP) is a key factor contributing to hyperglycemia in type 2 diabetes. This elevation in HGP is primarily caused by enhanced signaling through hepatic glucagon receptors (GCGRs). In a recent study, we demonstrated that selective inactivation of beta-arrestin-2 (barr2) in hepatocytes of adult mice leads to greatly increased hepatic GCGR signaling, resulting in profound impairments in glucose homeostasis. On the other hand, mic selectively lacking beta-arrestin-1 in hepatocytes did not show any changes in glucose homeostasis. Mice that over-expressed barr2 in hepatocytes displayed greatly reduced hepatic GCGR signaling and were protected against the metabolic deficits caused by the consumption of a high-fat diet. These new data strongly suggest that strategies aimed at enhancing hepatic barr2 activity could prove useful to suppress HGP for therapeutic purposes. (Zhu L, Rossi M, Cui Y, Lee RJ, Sakamoto W, Perry NA, Urs NM, Caron MG, Gurevich VV, Godlewski G, Kunos G, Chen M, Chen W, Wess J. Hepatic -arrestin-2 is essential for maintaining euglycemia. J Clin Invest 127, 2941-2945, 2017)